In automotive braking systems, a master cylinder converts driver exerted brake pedal force into a corresponding hydraulic pressure, which is proportioned among the front and rear brakes. In so-called power assisted braking systems, a vacuum booster is interposed between the pedal and the master cylinder to amplify the force applied to the master cylinder. The vacuum booster has access to engine vacuum, and amplifies the driver exerted force by controlling the pressure differential across one or more diaphragms coupled to the master cylinder. In a typical system, a vacuum chamber disposed on the master cylinder side of the diaphragm is coupled to engine vacuum, and an apply chamber disposed on the brake pedal side of the diaphragm is coupled to a valve that varies the pressure in the chamber between engine vacuum and atmospheric pressure. In these systems, the booster is said to reach a "run-out" condition when the apply chamber is at atmospheric pressure, since there can be no further amplification of the brake pedal force.
In more sophisticated braking systems, the force amplification of the vacuum booster is modified and/or supplemented by an electrically controlled pressure modulator. Control of the modulator requires knowledge of the pedal force and the operating state of the vacuum booster, particularly whether the booster is operating in a run-out condition. The conventional approach in this regard is to use dedicated sensors for measuring the necessary parameters. For example, a force sensor can be used to measure the pedal force, and a pressure sensor can be used to measure the apply chamber pressure, with an impending run-out condition being detected when the measured apply chamber pressure reaches atmospheric pressure. This approach, of course, entails the cost penalty of additional sensors, increasing the overall system cost.